High-efficiency light sources that offer a balanced and complete emission spectrum (white light) are used in the consumer, agricultural, and industrial market place. There are certain options available to these markets that meet the criteria of the light source being very efficient and offering a full spectrum light. Typical white light sources include high-pressure mercury lamps; metal halide lamps, high pressure xenon plasma lamps, or less frequently, sulfur plasma lamps. Xenon lamps are non-toxic but may emit ultraviolet light as well as visible light, and include xenon incandescent lamps used for automobile headlights, which do not emit ultraviolet light because the glass is engineered to filter out the ultraviolet, as well as xenon discharge lamps that can be expensive and that operate at very high pressures and can be dangerous if breached. In halide-containing lamps, the tungsten electrode material may tend to vaporize during operation of the lamp, but a recycling process known as the halogen cycle occurs in which the small amount of vaporized tungsten re condenses on the filament, prolonging life of the lamp.
Sulfur plasma lamps emit almost no ultraviolet light, which might otherwise waste energy, age plastic, or damage biological tissue, as can happen with mercury, tungsten filament lamps, or xenon lamps. Sulfur plasma lamps systems tend to be more complex and bulkier than, for example, metal halide lamps having just a small envelope with electrodes. Unlike metal halide lamps, in which bromides, chlorides, and mercury in the lamp emit a combination of atomic photoemission, sulfur plasma lamps produce molecular photoemission due to the presence of dimers and/or more complex sulfur molecules (S3, S8, etc.). Because plasma is very high energy the molecular state is dynamic and continuously changing and the molecular emission is over a broad spectrum, in contrast to the distinct spectral lines of atomic emission, thereby providing greater spectral uniformity over the 400-700 nanometer range of white light. The continuous sulfur plasma emission spectrum output nearly matches sunlight perfectly. Sulfur is benign and inexpensive with little environmental impact, and sulfur plasma lamps need not incorporate hazardous mercury as is used in other technologies. Sulfur plasma lamps are especially useful for providing light to plants. Plants, such as bananas, grown in indoor greenhouses with sulfur plasma lamps can yield more vegetation and fruit than plants grown with other traditional kinds of white light sources. Sulfur plasma lamps can be operated at high power levels and thereby illuminate very large spaces.
Sulfur plasma lamps currently available typically have no electrodes and are shielded and mounted in a microwave or EMF resonance cavity. One reliable method that has been found is to excite a sulfur plasma by using a microwave waveguide and resonance cavity system. A mesh is used in conjunction with the resonance cavity, similar to a microwave oven. The resonance cavity must be carefully tuned, and a magnetron is necessary for this method, but this method can provide a powerful lamp with a long lifetime, although there is a minimum power that is dependent on the type of magnetron that is available. Alternatively, microwave sources that are solid-state chips may be used instead of magnetrons in connection with sulfur or mercury plasma lamps. Another method uses a radio-frequency coil in near proximity to or around the perimeter of a hermetically sealed envelope containing sulfur and a buffer gas, as is done in connection with the Icetron induction light system. The use of a radio-frequency coil in near proximity to the sulfur induces excitation of the sulfur by way of electromagnetic induction. This technique involve complex technology that can be costly, and efficiency is limited due to the limits of coupling efficiency between the induction source and the contents of the lamp. Sulfur plasma lamps can be driven with electrodes if the lamp is spun at high speed to create centrifugal force that keeps sulfur away from the electrodes to prevent chemical reaction between the electrodes and sulfur. An attempt has been made to use metal electrodes coated with metal oxides for driving sulfur plasma lamps, with the intention that the metal oxide coating will prevent chemical reaction between the metal and the sulfur, but smooth and consistent operation of the lamp and the overall lifetime of the lamp is dependent on the oxide layer not being breached due to cyclical heating, aging, pinholing by passage of electricity in a small concentrated location, cracking due to thermal expansion mismatch between the oxide layer and metal, etc., and reliable and consistent discharge depends on the degree of predictability of electric current passing through the oxide layer.